Originally, it was the aqueous solutions of alkali metal hydroxides, above all soda lye, that were used as catalysts for the condensation reaction. The quality of the mineral wools prepared with such binders is appreciably spoiled under the effect of moisture. Even a neutralization of the alkaline fraction of these resins at the end of condensation, for example with dilute sulphuric acid, has not produced any appreciable improvement in the moisture resistance. This instability has been attributed to the presence in the finished material, even in the cured state, of soluble ash consisting of sodium oxide, sodium carbonate and/or sodium sulphate. Alkaline-earth metal hydroxides, in particular calcium hydroxide, have accordingly been employed as catalysts which have the advantage over the alkali metal hydroxides of being capable of being precipitated at the end of condensation, by the addition of dilute sulphuric or phosphoric acid, of carbon dioxide, of ammonium sulphate or of ammonium carbonate, and of being capable of being separated from the aqueous solution in the form of water-insoluble salts. With this operating technique, resins which are practically free from ash are obtained. Materials based on mineral fibres and prepared from these filtered resins have an excellent moisture resistance.
There is, however, a major disadvantage to the process described above: the insoluble precipitates thus formed must be removed by filtration; otherwise there is a risk of obstruction of the nozzles for distributing the resin when it is used. This filtration is a tedious operation which is accompanied by a loss of resin which is not insignificant; moreover, the dumping of the filter cake containing phenolic resins, and which can represent from 5 to 10% of the unfiltered reaction end product, is costly and nowadays constitutes a severe environmental problem.
Phenolic resins using alkaline-earth catalysis were therefore developed, in the preparation of which the alkaline-earth catalyst was neutralized in the form of insoluble species, but in such a way that its precipitation would not take place. Thus, use has been made of the introduction of sulphate and ammonium ions (EP 0 190 130--Rutgerswerke), but this, however, allows only a period of a few hours before the reappearance of calcium-containing precipitate, outweighs any possibility of transporting and stocking the resin and confines it to a utilization near the production site; or else the addition of the oxalic acid/protein emulsifier pair (U.S. Pat. No. 4,060,054--Monsanto), which retains the coalescence of the calcium precipitate in the form of oxalate but which, besides the presence of the protective colloid, may be detrimental to the quality of the agglomerates, causes a large increase in the viscosity of the product.
The unsolved problem is therefore that of obtaining industrially phenol/formalin resins using calcium catalysis, free of insoluble precipitate and therefore not requiring any filtration, and the stability of which is ensured over a period of 10 to 15 days corresponding to the actual stability of the resin between 10 and 15.degree. C., while the stability with regard to the precipitation of the calcium ions which is aimed at in the present invention is longer than 1 month.